Insulin-like growth factors in the brain and their potential clinical implications

Insulin-like peptides

The insulin-like peptides include insulin and the IGFs, IGF1 (or IGF-I) and IGF2 (or IGF-II). The IGFs consist of 3 domains that are in part homologous to that of human proinsulin; unlike the case of insulin, the mature IGF peptides are single chain polypeptides as their C domain is not removed during prohormone processing.^1,2 The brain expression of insulin, IGF1, and IGF2 is developmentally regulated.^1,2 Whereas all these 3 insulin-like peptides are expressed in the developing brain, evidence indicates that no insulin is produced by adult neural cells. IGF1 is produced by all cell types and its expression is highest perinatally, particularly in the neocortex, hippocampus, cerebellum, brainstem, hypothalamus, and spinal cord, and then decreases in the adult. IGF2 is produced at high levels during development and is the most abundant insulin-like peptide in the adult brain, with the highest levels of expression in the myelin sheaths, choroid plexus, leptomeninges, and hypothalamus.^1,2

Insulin-like peptide receptors

Signaling mechanisms

The biological actions of insulin-like peptides are mediated by 3 different types of receptors, with some functional overlap (figure). The insulin receptor is a homodimer formed by one α and one β subunit bridged by an intrinsic disulfide bond. Ligand binding to the α subunit activates the intrinsic tyrosine kinase activity located in the β subunit and subsequently initiates a cascade of phosphorylation that leads to recruitment of scaffold proteins, such as members of the insulin receptor substrate (IRS) family.³ The effects of IGF1 are primarily mediated by the IGF1 receptor (IGF1R). The IGF1R is a glycoprotein that forms homodimers but may also act as functional hybrid with the insulin receptor, suggesting a cooperation between IGF1 and insulin signaling.^1,2,4 Activation of the insulin or the IGF1 receptor triggers 2 canonical signaling pathways: the phosphoinositide 3-kinase (PI3K)–AKT (protein kinase B) and the RAS–extracellular signal-related kinase (ERK) pathway (figure). These pathways modulate gene transcription and activate multiple downstream kinases and phosphatases that affect protein synthesis, vesicular trafficking, cell growth and differentiation, metabolism, and resistance to oxidative stress.^1,–,4 The IGF1R is also detected in neuronal nuclei and can affect gene transcription directly by acting as a transcriptional modulator. Recent studies indicate that histone deacetylase 2, which is involved in epigenetic inhibition of transcription, colocalizes with IRS signaling components in postsynaptic glutamatergic neurons of the hippocampus.⁷

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Figure Overview of insulin-like peptide signaling in the brain

The insulin-like peptides include insulin and insulin-like growth factors (IGFs), including IGF1 and IGF2. The biological actions of insulin-like peptides are mediated by 3 different types of receptors: the insulin receptor (IR), IGF1 receptor (IGF1R), and IGF2 receptor (IGF2R). In the brain, these peptides act as paracrine signals via local production of IGF1 and IGF2 in neurons, astrocytes, microglia, and choroid epithelium; and endocrine signals, via access of circulating insulin and IGF1 across the blood–brain barrier. This involves the IGFR and low-density lipoprotein receptor–related protein (LRP) 1. IGF2 is directly synthesized by the choroid plexus. Bioavailability of IGF1 and IGF2 is regulated by their binding to IGF-binding proteins (IGFBPs). The effects of insulin and IGF1 are mediated by the insulin receptor, IGF1R, or hybrid receptors, via scaffold proteins of the insulin receptor substrate (IRS) family. This triggers 2 canonical signaling pathways: the phosphoinositide 3-kinase (PI3K)–AKT and the RAS–extracellular signal-related kinase (ERK) pathways; they activate multiple downstream effectors, including mTOR (mammalian target of rapamycin), glycogen synthase 3 kinase (GS3K), and Forkhead box O (FOXO) family of transcriptional activators. The IGF2R has multiple functions including endocytosis and lysosomal degradation of extracellular ligands; it also recruits G proteins and activates phospholipase C (PLC) and protein kinase C (PKC). BIM = BCL2-interacting mediator of cell death; ELK-1 = ETS-like transcription factor.

The effects of IGF2 are mediated primarily by the IGF2 receptor (IGF2R), which binds IGF2 with higher affinity than IGF1 and does not bind insulin.^1,2 This receptor is a single chain polypeptide with a short cytoplasmic domain that lacks tyrosine kinase activity. The IGF2 receptor is identical to the cation-independent mannose-6 phosphate (M6P) receptor; the IGF2/M6P receptor has multiple functions, including lysosomal enzyme trafficking, endocytosis, lysosomal degradation of extracellular ligands, and regulation of apoptosis. This receptor can also recruit G proteins and activate downstream signaling pathways, such as the phospholipase C and protein kinase C pathway. Other growth factors, such as transforming growth factor β, are physiologic ligands for the IGF2 receptor.^2,3

IGF binding proteins

There are 6 IGFBPs (IGFBP1–IGFBP6) that bind IGF1 and IGF2, but not insulin, and exert a tight control over IGF bioavailability and function.^2,8,–,10 These IGFBPs are produced by neurons, glial cells, choroid epithelia, and cerebral blood vessels, and are widely expressed in the neocortex, hippocampus, cerebellum, and spinal cord. The most abundant is IGFBP2, produced by astroglia and choroid plexus epithelial cells; IGFBP5 is produced by neurons.^2,10 IGFBPs transport and regulate the biological activity of IGFs by several mechanisms. They control IGF efflux from the vascular space, regulate IGF clearance, and modulate interaction of IGFs with their receptors.^2,8,–,10 The biological activity of IGFBPs is regulated both by posttranslational modifications such as glycosylation and phosphorylation, and by their differential localization in the pericellular and extracellular space. IGFBPs are further regulated by the presence of specific IGFBP proteases, which cleave the binding proteins and generate fragments with reduced or no binding affinity for the IGFs.^2,7,–,9

Passage across the blood–CSF barrier

Passage of circulating insulin and IGF1 across the blood–CSF barrier involves their constitutive transport by transcytosis mediated by the IGF1R and low-density lipoprotein receptor-related protein (LRP) 2. In contrast, IGF2 is directly synthesized by the choroid plexus. Once in the CSF, insulin-like peptides diffuse to periventricular areas, including the hypothalamus and the hippocampus; more distant areas are accessible only through a transport system into the brain, involving IGFBPs in the case of IGF1 and IGF2.¹

Passage across the BBB

Passage of circulating IGF1 across the BBB requires both the IGF1R and LRP1 and is relatively independent of IGF1 blood levels. There is evidence for a synaptic activity-dependent uptake of serum IGF1 by neurons (neurotrophic coupling), which occurs in parallel with the activity-dependent increase in local blood flow (neurovascular coupling).^1,11 These 2 coupling mechanisms share several mediators, including glutamate, released from presynaptic terminals; and prostaglandin E₂, arachidonic acid derivatives, and ATP, released from astrocytes, pericytes, and endothelial cells. These mediators stimulate the activity of matrix metalloproteinase-9, which cleaves IGFBP3, the major IGF1 carrier in the blood.¹¹ Once released from IGFBP3, IGF1 binds to IGF1Rs in endothelial cells and is then transported, gaining access to astrocyte end-feet and active neurons.¹

Development

Studies performed primarily in rodents demonstrate a developmental regulation of expression of insulin-like peptides, IGF receptors, and IGFBPs in the CNS.^1,–,4 Levels of these proteins are highest at the time of major CNS development and markedly decrease in the adult brain. IGF1 acts during all of the major phases of brain development; IGF1Rs are expressed in the ventral floor plate before the formation of the brain, and in the neocortex, hippocampus, cerebellum, hypothalamus, brainstem, and spinal cord during later developmental stages. IGF1 promotes proliferation of neural precursor cells; acts as a prosurvival signal for developing neurons; and facilitates subsequent steps of the differentiation of neurons, and of synaptogenesis and network formation in the hippocampus, neocortex, and olfactory bulb.^1,2,4 IGF1 signaling is also involved in the formation of the sensory organs of the eyes and ears and promotes maturation of astrocytes and oligodendrocytes.^6,12 The importance of IGF1 signaling in human brain development is reflected by the phenotypes caused by congenital IGF1 deficiency (intrauterine and postnatal growth retardation, microcephaly, and sensorineural deafness)¹³ or mutations of IGF1R (microcephaly and moyamoya disease).¹⁴ IGF1 also modulates neurogenesis and angiogenesis in the adult hippocampus.¹ Insulin, like IGF1, promotes synaptogenesis and astrogenesis.³ IGF2, together with IGF1, promotes oligodendrogenesis.^6,12

Synaptic plasticity and memory

Insulin-like peptides influence learning and memory by modulating synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), by several mechanisms.¹³ These include regulation of the synthesis and trafficking of glutamate and γ-aminobutyric acid receptor subunits, alteration of ion channel activity and neuronal excitability, and structural changes in the synapse. For example, in excitatory synapses, the insulin receptor substrate IRSp53 localizes at the postsynaptic densities via interactions of several scaffold proteins, and affects synaptic excitability and morphology^3,15 as well as dendritogenesis.¹⁶ Many of these effects involve PI3K-AKT and RAS-ERK signaling and regulation of protein phosphorylation, including that of AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid) and NMDA (N-methyl-d-aspartate) receptor subunits. For example, both IGF1 and insulin modulate LTD in the cerebellum and hippocampus by inducing phosphorylation of the GluR2 subunit and clathrin-mediated endocytosis of AMPA receptors. However, insulin may also facilitate LTP by enhancing insertion of the GluR1 subunits of the AMPA receptors in the synaptic membrane.³

Experimental evidence indicates that both aerobic and resistance exercise improves spatial memory; this is associated with increased expression of IGF1 in the hippocampus.¹⁷ Increased IGF2 signaling in the hippocampus may contribute to memory consolidation.¹⁸ The IGF2 gene is a target of the transcriptional activator CCAAT enhancer binding protein β and undergoes a temporally restricted upregulation during active phases of learning or memory retrieval.¹⁸

Central regulation of energy homeostasis

Systemic and brain insulin-like peptides participate in the crosstalk between the periphery and the brain for regulation of energy homeostasis.¹ Whereas circulating insulin does not appear to regulate glucose handling by neurons, it is a key modulator of the function of hypothalamic nuclei involved in homeostatic control of feeding behavior and carbohydrate and lipid metabolism.^19,20 Lack of insulin perinatally causes a defective hypothalamic circuitry, resulting in disturbed food intake in adulthood.¹

Neuroprotection

There is abundant experimental evidence that insulin and IGFs exert neuroprotective effects.^1,2 Both insulin and IGF1 activate the PI3K-AKT prosurvival pathway, which protects against apoptosis by inactivating proteins of the apoptotic machinery such as Bcl-2 family member Bad, glycogen synthase 3 kinase (GS3K), and Forkhead box O family of transcriptional activators.^1,3,4 IGF1 also increases Aβ clearance and blocks the action of proinflammatory cytokines and has a neuroprotective role in transgenic mouse models of Alzheimer disease (AD).²¹ In mammalian cell cultures, activation of the IGF1/AKT pathway protects neurons from polyglutamine-mediated toxicity; this involves the phosphorylation of huntingtin by AKT and its clearance by autophagy.^22,23 However, recent studies in invertebrates and mammals also suggest that, in certain circumstances, insulin and IGF1 may also exacerbate the abnormal protein processing that occurs both with aging and in neurodegenerative disorders.⁵ This apparent paradox may reflect differences in experimental models or context-dependent effects on different molecular mechanisms of disease.

Acute injury

Several types of injury elicit local increases in brain IGF signaling. For example, hypoxic injury elicits a coordinated induction of IGF1, IGF2, and IGFBP; in this setting, reactive microglia are the main source of IGFs, whereas astrocytes and neurons are the targets and overexpress IGF1Rs.²⁴ IGF1 is neuroprotective in animal models of stroke.²⁵ A recent study showed that patients with high plasma IGF1 or IGF1/IGFBP3 ratio measured within 6 hours after acute stoke had better neurologic and functional outcome at 3 months compared with patients with low IGF1 levels.²⁶ IGF1 may also exert neuroprotective effects in the setting of seizures; IGF1 administration following unilateral intrahippocampal administration of kainic acid in an animal model of temporal lobe epilepsy decreased seizure severity, protected against neurodegeneration, and abolished the resulting cognitive deficits.²⁷ However, adult IGF2 knockout mice showed less epileptiform activity and attenuated hippocampal neurodegeneration following kainic acid, suggesting that IGF2 may have a role in mechanisms that contribute to neurodegeneration in epilepsy.²⁸

Neurodegenerative disorders